1. Field of the Invention
The present invention relates to an image pickup apparatus such as an electronic still camera.
2. Related Background Art
In recent years, an electronic still camera has been developed which A/D (analog-to-digital) converts a signal output from a solid-state image pickup element such as a CCD (charge coupled device), and stores the digital data in a detachable semiconductor memory or a rotary floppy disk, thus allowing omission of a photographic film: example, a signal from a CCD sensor 201 which has color filters shown in FIGS. 2A to 2C in units of pixels is subjected to, e.g., a CDS (Correlated-Double-Sampling) process, a blanking pulse insert process, a preknee process, and the like in an analog process unit 202, and the processed signal is then converted into, e.g., 9-bit digital data by an A/D converter 203.
If this digital data is directly stored in a semiconductor memory 205, a capacity per frame is too large, and the number of images capable of being stored in a single semiconductor memory is reduced, resulting in an inconvenient electronic still camera.
A compression process unit 204 is normally arranged to compress digital data, and the compressed data is then stored in the semiconductor memory 205. Various compression methods are known. Of these methods, DPCM (differential pulse code modulation) and DCT (discrete cosine transformation) are often adopted.
Such a prior art suffers from two problems.
The first problem lies in arrangement of the A/D converter. In order t obtain satisfactory image quality free from quantization noise, the A/D converter 203 preferably has a larger number of quantization bits. In particular, when color filters are complementary color filters, since color signals are not base-band signals but modulated signals, at least 9 bits are necessary. This leads to a complicated arrangement of the A/D converter and an increase in cost of the apparatus.
The second problem lies in the compression process unit 204. Of course, the compression process unit is preferably omitted since a circuit arrangement can be simplified. However, if data is not compressed at all, the number of images which can be stored in the semiconductor memory is considerably decreased, as described above, resulting in inconvenience. Therefore, since the compression process unit must be separately arranged, this also leads to a complicated arrangement and an increase in cost.
In an image pickup apparatus such as a color video camera, color separation filters attached to a sensor often comprise complementary color filters rather than pure color filters.
The complementary color filters are advantageous in terms of a light utilization ratio, a band, and the like as compared with pure color filters.
Many sensors having complementary color filters have been developed mainly for movie cameras. FIG. 2C shows such a filter matrix. In order to obtain a still image for one frame using this sensor, read access is required twice per pixel, and exposure must be executed twice.